Navalny, who was convicted for embezzling funds, denounced the elections and is calling for a boycott: "The procedure in which we are invited to participate is not an election; it involves only Putin and those candidates whom he personally chose, who do not pose a slightest threat to him".
Videos posted on social media claim to show ballots being crammed with multiple votes by a sinle person.
Ella Pamfilova, head of the commission organizing the vote nationwide, has said any fraud will be stamped out.
Some 145,000 observers are monitoring the voting in the world's largest country, including 1,500 foreigners and representatives from opposition leader Alexei Navalny's political movement.
"Currently, the China-Russia comprehensive strategic cooperative partnership is at the best level in history, which sets an example for building a new type of global relations", Xi said in a congratulatory message to Putin, according to the official Xinhua news agency.More news: Korea prosecutors seek arrest of ex-president Lee
Ms Sobchak, a 36-year-old former TV star, told reporters in Moscow that the higher the support for Mr Putin in the vote, "the tougher the system" Russians will face in his new term.
Let's look at her portrayal of electoral democracy in Russian Federation as "dishonest, non-competitive, unfree elections", by starting with a historical analysis of politics under President Vladimir Putin.
NSA whistleblower Edward Snowden, who took asylum in Russian Federation in 2013, also shared a video he said showed election mishaps.
Earlier Sunday, there were widespread reports of ballot-box stuffing and forced voting, but complaints of voting irregularities will likely do little to undermine Putin, according to the AP.
Vladimir Putin attends a pre-election concert in Sevastopol, Mar 14, 2018.More news: Lewis Hamilton not close to final chapter
Putin thanked voters for their support at a victory rally and said Russian Federation had a great future ahead of it provided its people stayed united.
The doctor, who gave her name only as Yekaterina because of fears about repercussions, said she and her co-workers were told to fill out forms detailing not only where they would cast their ballots, but giving the names and details of two "allies" whom they promised to persuade to go vote.
Candidates are barred from campaigning on Saturday. Ukraine security forces blocked the Russian Embassy in Kiev and consulates elsewhere Sunday as the government protested the voting in Crimea, whose annexation is still not internationally recognized. Ukraine is also angry over Russian support for separatists in eastern Ukraine, where a deadly conflict continues. Navalny was prevented from running because of a criminal conviction widely viewed as politically motivated.
As U.S. authorities investigate alleged Russian hacking and other interference in President Donald Trump's 2016 election, Russian authorities have claimed that foreign powers are seeking to interfere in Sunday's vote.
Polls show that most Russians view the takeover of the Black Sea peninsula as a major achievement despite subsequent Western sanctions.More news: United States and South Korea announce start date for annual military exercises
Putin's challengers included Ksenia Sobchak, once known as Russia's "It Girl" and daughter of late Putin mentor Anatoly Sobchak.
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